1. Field of the Invention
The present invention relates to an optical device comprising a II-VI compound semiconductor and having a semiconductor laser or optical waveguide structure.
2. Description of the Related Art
In recent years, numerous attempts have been made to realize semiconductor lasers emitting blue-green light and made from II-VI compound semiconductor materials of ZnSe or ZnMgSSe, and various reports have been made thus far. Various structures of semiconductor devices have been proposed. Among them, a semiconductor laser device having a separate-confinement heterostructure (SCH structure) in which the threshold current density can be reduced has attracted attention.
Normally, a semiconductor laser device having the SCH structure can be manufactured by forming a first cladding layer, a first light guide layer, an active layer, a second light guide layer, and a second cladding layer successively on a compound semiconductor substrate. Heretofore, the light guide layers of semiconductor laser devices have been designed, taking account of only injection of currents, i.e., only the electrical characteristics. Therefore, the structure has not been optically optimized. That is, in order to enhance the light emission efficiency and to prevent electrons from recombining with positive holes without emitting light in the light guide layers, minimizing the thickness of the light guide layers has been considered to be preferable.
However, where the thickness of the light guide layers is reduced, light exudes through the light guide layers and spreads into the compound semiconductor substrate. In other words, the light is not completely confined by the light guide layers. The compound semiconductor substrate consisting of GaAs or the like absorbs a large portion of incident light. Therefore, a considerable portion of the light is absorbed by the compound semiconductor substrate. As a result, the threshold current density of the semiconductor laser device increases, thus lowering the differential efficiency. That is, it has been generally considered that there exists a conflicting relation between improvement of the electrical characteristics of the semiconductor laser device and improvement of the ability of the light guide layers to confine light.